Method and Apparatus for Flexible Circuit Cable Attachment

ABSTRACT

A method and apparatus for multiple flexible circuit cable attachment is described herein. Gold bumps are bonded on interconnection pads of a substrate to create a columnar structure and solder or conductive epoxy is dispensed on the flexible cable circuit. The substrate and flexible cable circuit are aligned and pressed together using force or placement of a weight on either the substrate or flexible cable circuit. Appropriate heat is applied to reflow the solder or cure the epoxy. The solder wets to the substrate pads, assisted by the gold bumps, and have reduced bridging risk due to the columnar structure. A nonconductive underfill epoxy is applied to increase mechanical strength.

CROSS-REFERENCE TO RELATION APPLICATION

This is application is a divisional of U.S. patent application Ser. No15/344,101, filed Nov. 4, 2016, the entire disclosure of which is herebyincorporated by reference.

FIELD OF INVENTION

The present invention relates generally to flexible circuit cables andin particular, to attachment of flexible circuit cables to a substrate.

BACKGROUND

Flexible circuit cables are widely used in chip to printed circuit board(PCB), chip to substrate, optical sub assembly to PCB, and PCB to PCBinterconnections. They provide high density signal routing capability ina limited space and flexible manner. However, previous methods forattaching the flexible circuit cables have a variety of disadvantages.

Direct soldering, for example, possible or preferable for singleflexible circuit cable attachment or multiple flexible circuit cableattachment when the flexible circuit cables are non-overlapping.However, in overlapping flexible circuit cable scenarios, the heatreflow of the solder will impact the chips or flexible circuit cablesthat are already attached on the substrate. For example, as shown inFIG. 1, pre-attached cable or flexible circuit cable Flex #1 will bedetrimentally affected when trying to attach flexible circuit cable Flex#2 using the direct soldering method. In scenarios when sequentialflexible circuit cables need to overlap with the pre-attached cable orflexible circuit cable, (Flex #1), it is nearly impossible to wet thesolder between the pads of the substrate and the second flexible circuitcable, (Flex #2). The gap created by the pre-attached cable or flexiblecircuit cable (Flex #1) interferes with placement of the second flexiblecircuit cable (Flex #2). The solder experiences difficulty in flowingbetween the flexible circuit cable and the substrate to createelectrical connectivity and a lasting reliable bond.

The Anisotropic Conductive Film (ACF), and/or Anisotropic ConductivePaste (ACP) approach is widely used in flexible circuit cable tosubstrate and chip to flexible circuit cable attachments for liquidcrystal display manufacturing. These processes also have severallimitations. For example, these processes require the electrical pads tobe embossed (raised) from the surface of the flexible circuit cable andsubstrate so that the conductive particles in the ACF or ACP can makecontact through compression to create electrical connectivity in the Zdirection. The ACF process also requires high thermal temperature tocure the film to create a bond. This high temperature can impact thechips or flexible circuit cable already attached on the substrate andtherefore overlapping flexible circuit cable attachment becomesdifficult with the ACF and/or ACP processes if there is a gap created bya previous attached flexible circuit cable or chips. Moreover, theconductive particle filled epoxies, traditionally used in ACP, usuallyhave a high resistance and result in a limited radio frequency (RF)bandwidth.

Flexible circuit cable attachment can also be done using conductiveepoxy. The conductive epoxy can be dispensed on the pads of the flexiblecircuit cable or substrate, prior to placement. Several issues existwith epoxy attachment including variance/planarity condition between theflexible circuit cable and substrate, under/over volume of epoxy, properpressure control, limited reworkability, bond strength, and higherresistance than solder. Additionally, added complexity due tooverlapping flexible circuit cables makes conductive epoxy even lessattractive.

In summary, there are many challenges in attaching overlapping flexiblecircuit cables to a substrate, interposer or other structure. Theattachment of the flexible circuit cable should not impact the assembledchips on the substrate or interposer. Any proposed method needs toovercome 1) the wetting issue between the flexible circuit cable and thesubstrate because of the gap formed by either previous assembledflexible circuit cables or chips, or the design of the flexible circuitcable and 2) co-planarity issue caused by pre-bending the flexiblecircuit cable, or unique shape such as U, S or open O shaped flexiblecircuit cables. The proposed method has to be operable in limitedspaces, account for signal RF bandwidth and achieve low signal crosstalkfor high bandwidth and high density signal trace on a single flexiblecable circuit. For example, flexible circuit cable attachments are usedin small assembly scenarios such as an optical sub assembly (OSA) usedin pluggable transceivers, (Small form-factor pluggable transceiver(SFP, SFP+, QSFP), C form-factor pluggable (CFP, CFP2), and the like).

SUMMARY

A method and apparatus for flexible circuit cable attachment isdescribed herein. Gold bumps are bonded on interconnection pads of asubstrate and solder or conductive epoxy is printed or dispensed on theflexible circuit cable. Multiple gold bumps can be bonded onto eachinterconnection pad to create a column to restrict the path of solder orepoxy. The substrate and flexible circuit cable are aligned and pressedtogether using force or placement of a weight on either the substrate orflexible circuit cable. Appropriate heat is applied to reflow the solderor cure the epoxy. The solder wets to the interconnection pads, (asassisted by the gold bumps), and have reduced bridging risk from thecolumn created by the multiple gold bumps on the interconnection pads.If conductive epoxy was printed or dispensed on the flexible circuitcable, then heat, ultraviolet (UV) light, or both can be applied to curethe epoxy between the flexible circuit cable and substrate. The epoxyhas a reduced risk of being squeezed and smeared to neighboring padsbecause of the gap and volume created by the gold bumps. For addedmechanical strength, a nonconductive underfill epoxy may be applied.Heat and capillary effect will draw the underfill epoxy in between theflexible circuit cable and substrate. The gold bumps create a standoffheight, assisting in the underfill path, solder wicking, and reducingrisk of bridging. Moreover, for high frequency applications, themultiple gold bumps result in improved radio frequency (RF) performance.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1 is a side view of direct soldering of a flexible circuit cable toa substrate with solder;

FIGS. 2A, 2B and 2C are examples of a substrate, a short form flexiblecircuit cable and a long form flexible circuit cable in accordance withan embodiment;

FIG. 3 is a top level flow diagram of attaching flexible circuit cablesto a substrate in accordance with an embodiment;

FIG. 4 is an example of a substrate with gold bumps in accordance withan embodiment;

FIG. 5 is an example of a substrate with one layer of gold bumps inaccordance with an embodiment;

FIG. 6 is an example of a substrate with two stacks of gold bumps inaccordance with an embodiment;

FIG. 7 are examples of different patterns for placing gold bumps on asubstrate in accordance with various embodiments;

FIG. 8 is a photograph of a six gold bump pattern on a substrate inaccordance with an embodiment;

FIG. 9A shows an example of an uncleaned flexible circuit cable inaccordance with an embodiment;

FIG. 9B shows an example of a cleaned flexible circuit cable inaccordance with an embodiment;

FIG. 10 is an example of a stencil used for printing solder on flexiblecircuit cable in accordance with an embodiment;

FIG. 11 is an example photograph of solder printed on the flexiblecircuit cable after a first reflow in accordance with an embodiment;

FIG. 12 is an example photograph of solder printed on the flexiblecircuit cable after a second reflow in accordance with an embodiment;

FIG. 13A shows an example of a flexible circuit cable being attached toa substrate in accordance with an embodiment;

FIG. 13B shows an example of a substrate being attached to a flexiblecircuit cable in accordance with an embodiment;

FIG. 14 is an example schematic illustrating an hot air rework system inaccordance with an embodiment;

FIG. 15 is an example photograph showing underfill in accordance with anembodiment;

FIG. 16 shows an example of a first flexible circuit cable attached to asubstrate in accordance with an embodiment;

FIG. 17 is an example schematic illustrating a bending tool inaccordance with an embodiment;

FIG. 18 shows an example of a second flexible circuit cable beingattached to a substrate in accordance with an embodiment;

FIG. 19 shows an example of first and second flexible circuit cablesattached to a substrate in accordance with an embodiment with twostacked gold bumps;

FIG. 20A is an example schematic illustrating a reflow fixture inaccordance with an embodiment;

FIG. 20B is an example schematic illustrating an exploded first flexiblecircuit cable assembly in accordance with an embodiment;

FIG. 20C is an example schematic illustrating an assembled firstflexible circuit cable assembly in accordance with an embodiment;

FIG. 21 is an example schematic illustrating a reflow fixture withthermocouples in accordance with an embodiment;

FIG. 22A is an example schematic illustrating a pickup stage and weightin accordance with an embodiment;

FIG. 22B is an example schematic illustrating a pickup stage, weight andassembled flexible circuit cable and substrate in accordance with anembodiment;

FIG. 23 is an example schematic illustrating a weight in accordance withan embodiment;

FIG. 24 is a top level flow diagram of attaching a first flexiblecircuit cable to a substrate in accordance with an embodiment;

FIG. 25 is an illustrative temperature profile for attaching a firstflexible circuit cable to a substrate in accordance with an embodiment;

FIG. 26 is a top level flow diagram of attaching a second flexiblecircuit cable to a substrate in accordance with an embodiment;

FIG. 27 is an illustrative temperature profile for attaching a secondflexible circuit cable to a substrate in accordance with an embodiment;and

FIG. 28 is an illustrative temperature profile for an underfill curingprocess in accordance with an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It is to be understood that the figures and descriptions of embodimentsof a method and apparatus for flexible circuit cable attachment havebeen simplified to illustrate elements that are relevant for a clearunderstanding, while eliminating, for the purpose of clarity, many otherelements found in typical vehicle systems. Those of ordinary skill inthe art may recognize that other elements and/or steps are desirableand/or required in implementing the present invention. However, becausesuch elements and steps are well known in the art, and because they donot facilitate a better understanding of the present invention, adiscussion of such elements and steps is not provided herein.

The non-limiting embodiments described herein are with respect to amethod and apparatus for flexible circuit cable attachment. The methodand apparatus for flexible circuit cable attachment may be modified fora variety of applications and uses while remaining within the spirit andscope of the claims. The embodiments and variations described herein,and/or shown in the drawings, are presented by way of example only andare not limiting as to the scope and spirit. The descriptions herein maybe applicable to all embodiments of the method and apparatus forflexible circuit cable attachment including, for example but not limitedto, chip to printed circuit board (PCB) interconnection, chip tosubstrate interconnections, optical sub assembly to PCBinterconnections, and PCB to PCB interconnections, for example.

Referring now to the drawings wherein similar reference numerals referto similar elements across the several views, a method and apparatus forflexible circuit cable attachment is described. The embodimentsdescribed herein provide a solution for connecting flexible circuitcables and in some embodiments when multiple flexible circuit cables areoverlapping, for example.

Described herein is an illustrative method which provides multiple goldbumps on interconnection pads located on a substrate. The gold bumpsovercome a gap or variance between the substrate and flexible circuitcables when multiple flexible circuit cables are attached to thesubstrate and in particular, when the flexible circuit cables areoverlapping. Moreover, the gold bumps provide increased wetting ofsolder from the flexible circuit cables to the substrate, a reduced riskof bridging or smearing, and a minimum bond gap, which allows for theuse of underfill to increase mechanical strength. The flexible circuitcables also utilize a double solder bumping in some embodiments toincrease the solder volume and to help compensate for the gaps betweenthe flexible circuit cables and the substrate. A weight is used to keepthe flexible circuit cables in close contact with the substrate duringreflow and reduce the risk of open solder joints or head-in-pillow.Multiple fixtures were designed and developed as described herein.

In general and as further described herein below, interconnection padson a substrate are provided with 1 or more gold (or copper) bumps. Forexample, each interconnection pad can have 3 to 9 gold (or copper)bumps. The bumps can be, for purposes of illustration, in a circularform or shape on the interconnection pad. Note, there is no limitationon the type of substrate, it can be silicon or glass interposer, LCDglass, or a regular PCB, another flex circuit, or even a chip with padsthat can be gold (cu) bumped. A stencil print is used to deposit solderor dispense conductive epoxy on the flex circuit cable to be attached.The flexible circuit cable and substrate are aligned to each other andplaced together. Pressure is applied through a force or a weight on topof the substrate or flexible circuit cable, whichever is on the top.

Heat is applied to reflow the solder or cure the epoxy. Solder will flowto the substrate with wetting assisted by the gold bumps and havingreduced bridging risk from the columns created by the multiple gold (orcopper) bumps on the substrate. In the event conductive epoxy wasprinted on the flexible circuit cable, then heat, ultraviolet (UV)light, or both can be applied to cure the epoxy between the flexiblecircuit cable and substrate. The epoxy has a reduced risk of beingsqueezed and smeared to neighboring pads because of the gap created bythe gold (or copper) bumps. Once cooled, for increased mechanicalstrength, a nonconductive underfill epoxy is applied. Heat and capillaryeffect will draw the underfill epoxy in between the flexible circuitcable and substrate. After properly curing the underfill epoxy, theweight and or the pressure applied is removed.

The above can be repeated for each flexible circuit cable to be attachedonto the substrate. The additional flexible circuit cable can bepre-bent with a bending jig to reduce downstream assembly complexity.The gold bumps on the substrate or flexible circuit cable can compensatefor the standoff height created by the first flexible circuit cable,assisting in the underfill path, solder wicking, and reducing the riskof bridging. X-ray imaging, connectivity testing, and shear and pulltesting can be performed to verify the mechanical integrity of themultiple flexible circuit cable attachments.

FIG. 2A is an illustrative example of a substrate 200. The substrate 200can include components 205 and interconnection pads 210. The substratecan be, but is not limited to, single fused silicon (Si) glass substrateor any other type of similar material. Although the description hereinis with respect to a substrate, the method can be applied tointerposers, silicon or glass interposer, liquid crystal display (LCD)glass, a printed circuit board (PCB), another flexible circuit, a chipwith pads that can be gold (copper) bumped and other interface modules.

FIG. 2B is an illustrative example of a flexible circuit cable 220 andin particular, a short form flexible circuit cable. The flexible circuitcable 220 has interconnection pads 225. FIG. 2C is an illustrativeexample of another flexible circuit cable 230 and in particular, a longform flexible circuit cable. The flexible circuit cable 230 hasinterconnection pads 235. The flexible circuit cables illustrated inFIGS. 2B and 2C are illustrative and other forms and shapes may be usedwithout departing from the scope of the claims.

FIG. 3 is a top level flow diagram 300 of attaching flexible circuitcables to a substrate in accordance with an embodiment. Gold bumps arewire bonded to a substrate (305). In an embodiment, the bumps can beultrasonically attached to the substrate. Any attachment method may beused that results in diffusion of the bump material into the substrate.Although the description herein is with respect to gold bumps, othermaterials may be used, such as for example, copper and aluminum. FIG. 4shows a substrate 400 that has gold bumps 405 on interconnection pads410 in accordance with an embodiment. FIG. 5 is an example of asubstrate 500 with one stack of gold bumps 505 in accordance with anembodiment. FIG. 6 is an example of a substrate 600 with two stacks ofgold bumps in accordance with an embodiment. A first stack of gold bumps605 is wire bonded to interconnection pads that correspond to placementof a first and second flexible circuit cable and a second stack of goldbumps 610 is wire bonded to interconnection bonds that correspond toplacement of the second flexible circuit cable. As stated herein, thegold bumps provide additional volume, standoff height to assist inproviding an underfill path, solder wicking and reduced risk of bridgingbetween interconnection pads. In addition, the bumps can serve to set a“standoff height” between the substrate and flexible circuit cable toensure that solder is not forced out under attachment pressure.

FIG. 7 illustrates different patterns for placing gold bumps on asubstrate in accordance with various embodiments including, but notlimited to, one gold bump, two gold bumps, three gold bumps, four goldbumps, five gold bumps and six gold bumps. The gold bumps can be laidout in a variety of patterns and shapes, including but not limited to,circular patterns. The number of gold bumps is illustrative and candepend on the nature of substrate, available area, interconnection padsize and other similar considerations. FIG. 8 is a photograph of a sixgold bump pattern on a substrate in accordance with an embodiment.

Referring back to FIG. 3, the flexible circuit cables are cleaned (310).Each flexible circuit cable is placed or dipped into denatured alcoholfor an appropriate amount of time to clean off contaminants. Theplacement in the denatured alcohol can be for approximately 15 seconds,for example. The flexible circuit cables are then placed in tarnishremover for an appropriate amount of time. The placement in the tarnishremover can be for approximately 3 minutes, for example. The tarnishremover with the flexible circuit cables can be agitated to encouragethe cleansing process. The flexible circuit cables are then placed intoa bag of denatured alcohol and the bag containing the flexible circuitcables are then positioned in an ultrasonic bath for an appropriateamount of time. The placement in the ultrasonic bath can be forapproximately 10 minutes, for example. The flexible circuit cables arethen removed and allowed to air dry. FIG. 9A shows an example of anuncleaned flexible cable circuit and FIG. 9B shows an example of acleaned flexible circuit cable in accordance with an embodiment.

Referring back to FIG. 3, solder bumps are printed on a first flexiblecircuit cable (315). For purposes of illustration only, the firstflexible circuit cable is a small form flexible circuit cable. In anembodiment, the solder paste is Tin-Bismuth, which has a low temperaturemelting point. A variety of solder pastes can be used as the selectionof the solder paste is dependent upon the application and other similarfactors. Although the description herein is with respect to a doublebumping solder method, the number of solder bumps, including the use ofa single solder bump, is dependent upon area, size and other similarfactors. In another embodiment, solder or conductive epoxy are notnecessary if using a diffusion bonding process such as thermalcompression or ultrasonic bonding.

The solder paste is applied to the flexible circuit cable using astencil. FIG. 10 is an example of the stencil used for printing solderon the flexible circuit cable in accordance with an embodiment. Theflexible circuit cable with the solder paste is then placed onto ahotplate at a predetermined temperature. The predetermined temperaturecan be 250° C., for example. The flexible circuit cable can be removedfrom the hotplate once the solder paste has fully reflowed (i.e. melted)and allowed to cool. The flexible circuit cable is placed into denaturedalcohol and an ultrasonic bath for a predetermined or appropriate amountof time. The predetermined or appropriate amount of time may be 10minutes, for example. In an embodiment where double bumping is needed,solder paste is applied over the reflowed solder paste to additionalsolder volume using the stencil. FIG. 11 is an example photograph ofsolder printed on the flexible circuit cable after a first reflow inaccordance with an embodiment and FIG. 12 is an example photograph ofsolder printed on the flexible circuit cable after a second reflow inaccordance with an embodiment.

Referring back to FIG. 3, the first flexible circuit cable is placedonto a hot air reflow system (HARS) fixture (320). The substrate and aweight are placed onto a pickup stage. The flexible circuit cable andsubstrate are positioned as shown in either FIG. 13A or 13 B. Inparticular, FIG. 13A shows an example of the flexible circuit cable 1300being attached to a substrate 1305 where the flexible circuit cable 1300is above the substrate 1305. Fixtures 1310 and 1315 are used to hold andpress the flexible circuit cable 1300 and the substrate 1305 together,respectively. FIG. 13B shows an example of a substrate 1320 beingattached to a flexible circuit cable 1325 where the substrate 1320 isabove the flexible circuit cable 1325. Fixtures 1330 and 1335 are usedto hold and press the flexible circuit cable 1320 and the substrate 1325together, respectively. In an embodiment, the weight is cantilevered offof the flexible circuit cable to increase compression and bonding. Afterappropriate placement, the HARS sequence for the first flexible circuitcable and substrate in then initiated (325). A more detailed descriptionof the HARS process is presented with respect to FIG. 24.

FIG. 14 is an example schematic illustrating the HARS fixture 1400 inaccordance with an embodiment. In general, and as further describedherein below, the HARS fixture 1400 is designed to control thetemperature in predetermined temperature ranges so that the firstflexible circuit cable and the substrate are bonded together withoutaffecting any electronic components or the integrity of the substrate.

In particular, in an embodiment using solder, the HARS sequence will usethe appropriate amount of heat as described herein below to reflow thesolder, and the solder will flow to the substrate through the goldbumps, and be contained by the column created by the multiple gold bumpson the substrate.

Although the description herein is with respect to solder, a conductiveepoxy can be used. In the event conductive epoxy is printed or dispensedon the flexible circuit cable, then heat, ultraviolet (UV) light, orboth is to cure the epoxy between the flexible circuit cable andsubstrate. The epoxy has a reduced risk of being squeezed and smeared toneighboring pads because of the gap created by the gold bumps. That is,the epoxy is contained by the column created by the multiple gold bumpson the substrate.

After completion of the HARS sequence, the first flexible circuit cableand the substrate are allowed to cool, and then underfill is applied andallowed to cure to increase the mechanical stability (330). In anembodiment, the underfill is a non-conductive underfill epoxy. Theunderfill is applied to the flexible circuit cable edge next to thesubstrate. Heat and capillary effects draw the underfill epoxy betweenthe flexible circuit cable and the substrate. Application of theunderfill epoxy is stopped if a fillet is formed around the edge of theflexible circuit cable. The heat is applied through a heat block builtinto the holding fixture, or through convention heating. FIG. 15 is anexample photograph showing underfill in accordance with an embodiment.The first flexible circuit cable attachment to the substrate may then beexamined using, for example, X-rays (335).

FIG. 16 shows an example of a first flexible circuit cable attached to asubstrate in accordance with an embodiment. In particular, the firstflexible circuit cable 1600 is attached to a part of the substrate 1605using a single row of gold bumps 1610.

Referring back to FIG. 3, solder bumps are printed on a second flexiblecircuit cable (340). For purposes of illustration only, the secondflexible circuit cable is a large form flexible circuit cable. In anembodiment, the solder paste is Tin-Bismuth, although a variety ofsolder pastes can be used as the selection of the solder paste isdependent upon the application and other similar factors. The solderpaste is applied to the flexible circuit cable using a stencil. Theflexible circuit cable with the solder paste is then placed onto ahotplate at a predetermined temperature. The predetermined temperaturecan be 250° C., for example. The flexible circuit cable can be removedfrom the hotplate once the solder paste has fully reflowed (i.e. melted)and allowed to cool. The flexible circuit cable is placed into denaturedalcohol and an ultrasonic bath for a predetermined or appropriate amountof time.

The second flexible circuit cable needs to be bent at a predeterminedangle with respect to the interconnect pads and the rest of the secondflexible circuit cable. The predetermined angle is sufficient to clearthe second flexible cable circuit with respect to the first flexiblecircuit cable or other component carrying module. In an embodiment, thepredetermined angle can be 35°. In another embodiment, the predeterminedangle is between 35° and 60°. This may be done using a bending tool 1700as shown in FIG. 17. In particular, a flexible circuit cable 1705 isplaced in the bending tool 1700. The flexible circuit cable and thebending tool are then placed into an oven at a predetermined temperaturefor a predetermined amount of time. In an illustrative example, thepredetermined temperature is 60° C. and the predetermined time is 1hour. As before, if double bumping is needed, solder paste is appliedover the reflowed solder paste to additional solder volume using thestencil. As before, solder paste is illustrative and conductive epoxymay be used.

Referring back to FIG. 3, the second flexible circuit cable is placedinto a hot air reflow system (HARS) fixture (345). The substrate and aweight are placed onto a pickup stage. The flexible circuit cable andsubstrate are positioned as shown in either FIG. 13A or 13B. FIG. 18shows an example of a second flexible circuit cable 1800 being attachedto a substrate 1805 in accordance with an embodiment. In particular, afixture 1810 is placed on top of the second flexible circuit cable 1800and a fixture 1815 is used to hold and press the substrate 1805.

After appropriate placement, the HARS sequence for the second flexiblecircuit cable and substrate in then initiated (350). A more detaileddescription of the HARS process is presented with respect to FIG. 26. Asdescribed above, after completion of the HARS sequence, the secondflexible circuit cable and the substrate are allowed to cool, and thenunderfill is applied and allowed to cure to increase the mechanicalstability (355). The second flexible circuit cable attachment to thesubstrate may then be examined using, for example, X-rays (360).

FIG. 19 shows an example of first and second flexible circuit cablesattached to a substrate in accordance with an embodiment. In particular,a first flexible circuit cable 1900 is attached to a part of thesubstrate 1905 using a single stack of gold bumps 1910 and a secondflexible cable circuit 1915 is attached to a part of the substrate 1905using a double stack of gold bumps 1920.

Referring back to FIG. 14 and also to FIGS. 20-23, described herein isthe HARS fixture 1400, and the various fixtures needed to implement theHARS processes for the first and second flexible circuit cables. TheHARS fixture 1400 includes a top heater 1405, a bottom heater 1410, afixture support leg 1415, sheet metal 1420 and polyimide tape 1425. Thepolyimide tape 1425 is used to cover all of the air holes on the bottomheater 1405 except near the center. This forces all of the air flowtowards the center of the bottom heater 1405. The sheet metal 1420contains the hot air by creating a chimney-type effect and directing theair towards a reflow fixture 1430 that also includes the flexible cablecircuits. The reflow fixture 1430 is supported by the fixture supportleg 1415, which is a perpendicular beam, suspended above the bottomheater 1410. The top heater sits a predetermined distance above thereflow fixture 1430 and provides hot N₂ gas. The top heater 1405 heightand temperature are necessary to control solder temperature and the N₂assists with solder wetting. The predetermined distance of the topheater 1405 can vary depending on application and materials and can be,for purposes of illustration only, 25 mm.

FIG. 20A is an example schematic illustrating a reflow fixture 2000 inaccordance with an embodiment. Moreover, FIG. 20A illustrates how afirst flexible circuit cable 2005 and a second flexible circuit cable2010 is positioned on the reflow fixture 2000. The reflow fixture 2000has a vacuum line 2015 that is used to secure the first flexible circuitcable 2005 and the second flexible circuit cable 2010 during reflow.Various holes 2020 were made to assist with fixture pre-heating from abottom heater. The reflow fixture 2000 can be made from a number ofsuitable materials including for purposes of illustration, aluminum. Thesubstrate is blocked from all heated airflow to minimize shifting duringreflow. The reflow fixture 2000 is also designed to fit the HARS fixtureposition clamps, (i.e. the fixture support leg 1415 in FIG. 14), toensure consistent placement and position as shown in FIG. 14. FIG. 20Bis an example schematic illustrating an exploded first flexible circuitcable assembly 2030 in accordance with an embodiment. The first flexiblecircuit cable assembly 2030 includes a reflow fixture 2035 as describedherein above, a flexible circuit cable 2040, a substrate 2045 and aweight 2050. FIG. 20C is an example schematic illustrating an assembledfirst flexible circuit cable assembly 2060 using the elements describedabove in accordance with an embodiment.

FIG. 21 is an example schematic illustrating a reflow fixture 2100 withthermocouples 2105 in accordance with an embodiment. In particular, eachposition marked by an “X” in FIG. 21 indicates thermocouple locationsfor thermal profiling. To simulate thermal mass for assembly, thesubstrate and a Tungsten block, (which is being used as the weight andfurther described herein below), are both placed, (as shown in FIGS. 20Band 20C), when determining the thermal profile. A thermocouple wasplaced on the reflow fixture 2100 as a conditioning mark to triggerchanges with the bottom and top heaters.

FIG. 22A is an example schematic illustrating a HARS pickup stage 2200,weight 2205 and substrate 2210 in accordance with an embodiment. TheHARS pickup stage 2200 is designed to prepare the substrate 2210 and theweight 2205, (e.g., a Tungsten block), to be picked simultaneously andin a consistent position. FIG. 22B is an example schematic illustratinga pickup stage 2250, weight 2255, and an assembled flexible circuitcable 2265 and substrate 2260 in accordance with an embodiment.

FIG. 23 is an example schematic illustrating a weight 2300 in accordancewith an embodiment. In addition to the vacuum channel securing theflexible circuit cables to the reflow fixture, a weight is applied ontothe substrate during the hot air reflow process. The main part of theweight 2300, can be, but is not limited to, a Tungsten block. Othermetals may be used depending upon application. The weight 2300 consistsof alternating layers of double-sided polyimide 2305 andPolytetrafluoroethylene (PTFE) 2310 which are added to provide thermalinsulation between the tungsten block 2315 and a substrate. The outerlayer consists of single-sided polyimide layer 2320. To provide suction,hole 2325 was drilled through the tungsten block 2300 and insulatinglayers 2305, 2310 and 2320. The hole can be, but is not limited to, a 2mm hole. This allows the weight 2300 and the substrate to be picked andplaced simultaneously by a HARS pickup tube, (not shown), as is commonlyknown. In an embodiment, the weight 2300 may have a 33 g tungsten block,0.14 mm thick double-sided polyimide tape (2×), 0.14 mm thick PTFE (2×)and 0.03 mm thick single-sided polyimide (1×).

FIG. 24 is a top level flow diagram 2400 of attaching a first flexiblecircuit cable to a substrate in accordance with an embodiment. Asubstrate and weight are picked up from a HARS pickup stage (2405). Thesubstrate and a flexible circuit cable, (e.g. small form flexiblecircuit cable), are aligned and placed in a HARS fixture (2410).Referring now also to FIG. 14, a bottom heater 1410 and a top heater1405 are turned on and stabilized to 225° C. and 150° C., respectively(2415). The temperatures are illustrative and other temperatures may beappropriate depending upon application. When reflow fixture 1430 reachesa predetermined temperature, (for purposes of illustration this may be166° C. but may vary depending upon application), the bottom heater 1410temperature is ramped down and stabilized at another predeterminedtemperature, (which may be 178° C. but can vary depending uponapplication) (2420). A timer is set for two minutes to increase liquidustime (2425). The timer time is illustrative and other times may be usedbased application and materials used. After expiration of the timer, thebottom heater 1410 and top heater 1405 are set to room temperature and acooling boost is induced from the top heater 1405 (2430). When reflowfixture 1430 temperature reaches 138° C., external cooling fans areturned on (2535). When reflow fixture 1430 temperature reaches 80° C.,the external cooling fans, bottom heater 1410, and top heater 1405 areturned off (2440).

FIG. 25 is an illustrative temperature profile for attaching the firstflexible circuit cable to the substrate in accordance with anembodiment. The temperatures and temperature ranges described hereinallow fast attainment of the reflow temperature before flux burns out inthe solder. The flux reduces oxides and the oxides need to be low toachieve sufficient binding. Moreover, as illustrated in FIG. 25, themethods described herein allow for operating in a narrow thermal windowso as to achieve the binding without damaging the components. Thetemperatures stated in the embodiment herein are illustrative for theapplication and materials used herein. Other temperatures may be usedwithout departing from the scope of the claims.

FIG. 26 is a top level flow diagram 2600 of attaching a second flexiblecircuit cable to a substrate in accordance with an embodiment. Asubstrate and weight are picked up from a HARS pickup stage (2605). Thesubstrate and a flexible circuit cable, (e.g. large form flexiblecircuit cable), are aligned and placed in a HARS fixture (2610).Referring now also to FIG. 14, a bottom heater 1410 and a top heater1405 are turned on and stabilized to 225° C. and150 ° C., respectively(2615). The temperatures are illustrative and other temperatures may beappropriate depending upon application. When reflow fixture 1430 reachesa predetermined temperature, (for purposes of illustration this may be162° C. but may vary depending upon application), the bottom heater 1410temperature is ramped down and stabilized at another predeterminedtemperature, (which may be 182° C. but can vary depending uponapplication) (2620). A timer is set for two minutes to increase liquidustime (2625). The timer time is illustrative and other times may be usedbased application and materials used. After expiration of the timer, thebottom heater 1410 and top heater 1405 are set to room temperature and acooling boost is induced from the top heater 1405 (2630). When reflowfixture 1430 temperature reaches 138° C., external cooling fans areturned on (2535). When reflow fixture 1430 temperature reaches 80° C.,the external cooling fans, bottom heater 1410, and top heater 1405 areturned off. FIG. 27 is an illustrative temperature profile for attachingthe second flexible circuit cable to the substrate in accordance with anembodiment. The temperatures stated in the embodiment herein areillustrative for the application and materials used herein. Othertemperatures may be used without departing from the scope of the claims.

As stated herein above, immediately after each flexible circuit cableattachment, an epoxy underfill is applied to add mechanical strength.The epoxy underfill can be cured while mounted on the HARS fixture toreduce risk to the flexible circuit cable attachment for the secondflexible circuit cable attachment. A single droplet of the underfillmaterial can be used for the first or small flexible circuit cableassembly and two droplets can be used required for the second or largeflexible circuit cable assembly. The number of drops is illustrative andmay vary in accordance with application and materials used. Referringback to FIG. 15, the arrows indicate underfill material post-cured. Inan illustrative embodiment, the underfill is a ball grid array (BGA)underfill and can be applied at 80° C. and cured at 130° C. for morethan 8 minutes. The materials, time and temperatures are illustrativeand other values may be used without departing from the scope of theclaims. It is noted that the top heater may not be needed for underfillcuring.

The methods and apparatus described herein can achieve flexible circuitcable attachment even when the flexible circuit cable has an irregularshape. For example, when the interconnection pads on the flexiblecircuit cable are located on a U shaped area. The methods overcome theco-planarity issue of the pads on a flexible circuit cable if theflexible circuit cable is pre-bent and the co-planarity is lost. Themethods provide multiple flexible circuit cable attachment, and theflexible circuit cables can overlap each other. Moreover, the methodscan overcome the space gaps created by pre-attached chips or flexiblecircuit cables.

The attachment temperature is low and will not impact pre-attached chipsor flexible circuit cables. The methods reduce the attachment resistancebetween the flexible circuit cable and substrate with multiple goldbumps (or cu pillars), and therefore provides wider RF bandwidth andbetter signal integrity of the interconnection than other attachmenttechniques. For example, solderable conductive epoxy SMT138 E has anelectrical resistance of 1030 μΩ.cm, which is about 10 times larger thanthat of the gold at 2.44 μΩ.cm.

The methods can help improve crosstalk performance by using more thanone flexible circuit cable instead of routing signals through a singleflexible circuit cable. When high speed signals are placed too close toeach other on a flexible circuit cable, crosstalk between the signallanes on the same flexible circuit cable will occur. Therefore, it is anadvantage to use more flexible circuit cables to transmit signals thatrequire very low cross talk, such as the high speed signals to drive theoptical transmitters and the high speed signals from the opticalreceivers. The methods provide a high density, high throughput, widebandwidth signal fan-out solution from a small substrate or interposer.The method can reduce the pad size and pitch on the substrate andflexible circuit cable, and therefore increase interconnection densitygreater than any of the existing flexible circuit cable attachmentsolutions.

The method increases attachment reliability because the solder orconductive epoxy are contained by the gold bumps, and will not smearinto neighboring pads. The method reduces attachment process timebecause soldering and underfill can be performed at the same time.

In summary, the gold bumping allows for: 1) flexible circuit cableattachment with limited co-planarity; 2) attachment of irregular shapedflexible circuit cables; 3) multiple flexible circuit cable attachmentsto same substrate (overcoming spacing created by stacking flexiblecircuit cable); 4) can use low temperature and pressure processes; 5)reduces electrical resistance between substrate and flexible circuitcable; 6) increased design flexibility; and 7) increase RF performancefor high frequency applications.

In general, a method for flexible circuit cable attachment includesbonding a plurality of gold bumps to each interconnection pad of aplurality of interconnection pads on a substrate to create columns ateach interconnection pad. A bonding material is dispensed on a firstflexible circuit cable. The substrate and the first flexible circuitcable are aligned and forcibly pressed together, where the columnrestricts dispersion of the bonding material. A first set ofpredetermined levels of heat is applied to promote the bonding materialto bond between the substrate and the first flexible circuit cable. Inan implementation, the method includes bending a second flexible circuitcable with respect to the first flexible circuit cable, bonding anotherplurality of gold bumps to each interconnection pad of another pluralityof interconnection pads on the substrate to create columns at eachinterconnection pad, dispensing a bonding material on the secondflexible circuit cable, aligning and forcibly pressing the substrate andthe second flexible circuit cable together, and applying a second set ofpredetermined levels of heat to promote the bonding material to bondbetween the substrate and the second flexible circuit cable. In animplementation, the first flexible circuit cable and the second flexiblecircuit cable are overlapping. In an implementation, the firstpredetermined levels of heat and the second predetermined levels of heatare controlled to affect bonding between the substrate and the firstflexible cable and between the substrate and the second flexible circuitcable without affecting electronic components. In an implementation, thesecond predetermined levels of heat are controlled to affect bondingbetween the substrate and the second flexible cable without affectingelectronic components and the bonding between the substrate and thefirst flexible circuit cable. In an implementation, a weight is used toforcibly press at least one of the substrate and first flexible circuitcable together and the substrate and second flexible circuit cabletogether. In an implementation, the weight is cantilevered off of atleast the first flexible circuit cable to enhance compression andbonding. In an implementation, the bonding material is dispensed atleast twice on the first flexible circuit cable. In an implementation,the method includes applying an underfill between the substrate and thefirst flexible circuit cable to provide mechanical strength. In animplementation, the plurality of gold bumps are laid out on eachinterconnection pad in a predetermined pattern.

In general, a system for attaching flexible circuit cables includes asubstrate including a first set of interconnection pads, where aplurality of gold bumps are bonded to each interconnection pad to createa columnar structure. The system includes at least one flexible circuitcable, where a bonding material is dispensed on the at least oneflexible circuit cable, an alignment device to align the substrate andthe at least one flexible circuit cable, a weight to forcibly presstogether the substrate and the at least one flexible circuit cable, anda hot air reflow system (HARS) to apply a first set of predeterminedlevels of heat to promote the bonding material to bond between thesubstrate and the at least one flexible circuit cable, wherein thecolumnar structure restricts dispersion of the bonding material. In animplementation, another plurality of gold bumps are bonded to anotherset of interconnection pads to create additional columnar structures. Inan implementation, the system further includes a bending tool, at leastanother flexible circuit cable, where the bending tool bends the atleast another flexible circuit cable with respect to the at least oneflexible circuit cable and where bonding material is dispensed on the atleast another flexible circuit cable, the alignment device aligning thesubstrate and the at least another flexible circuit cable, the weightforcibly pressing the substrate and the at least another flexiblecircuit cable together and the HARS applying a second set ofpredetermined levels of heat to promote the bonding material to bondbetween the substrate and the at least another flexible circuit cable.In an implementation, the at least one flexible circuit cable and the atleast another flexible circuit cable are overlapping. In animplementation, the first predetermined levels of heat and the secondpredetermined levels of heat are controlled to affect bonding betweenthe substrate and the at least one flexible cable and between thesubstrate and the at least another flexible circuit cable withoutaffecting electronic components. In an implementation, the secondpredetermined levels of heat are controlled to affect bonding betweenthe substrate and the at least another flexible cable without affectingelectronic components and the bonding between the substrate and the atleast one flexible circuit cable. In an implementation, the weight iscantilevered off of at least the at least one flexible circuit cable andthe at least another flexible circuit cable to enhance compression andbonding. In an implementation, the bonding material is dispensed atleast twice on at least one of the at least one flexible circuit cableand the at least another flexible circuit cable. In an implementation,an underfill is applied between the substrate and the at least oneflexible circuit cable and between the substrate and the at leastanother flexible circuit cable to provide mechanical strength. In animplementation, the plurality of gold bumps and the another plurality ofgold bumps are laid in a predetermined pattern. In an implementation,the HARS includes a plurality of heaters to provide the first set ofpredetermined levels of heat and the second set of predetermined levelsof heat.

It is to be understood that the present invention is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the following claims. Additionally, although thefeatures and elements of the present application are described in theexample embodiments in particular combinations, each feature or elementcan be used alone (without the other features and elements of theexample embodiments) or in various combinations with or without otherfeatures and elements of the present application.

What is claimed is:
 1. A system for attaching flexible circuit cables,comprising: a substrate including a first set of interconnection pads,wherein a plurality of gold bumps are bonded to each interconnection padto create a columnar structure; at least one flexible circuit cable,wherein a bonding material is dispensed on the at least one flexiblecircuit cable; an alignment device to align the substrate and the atleast one flexible circuit cable; a weight to forcibly press togetherthe substrate and the at least one flexible circuit cable; and a hot airreflow system (HARS) to apply a first set of predetermined levels ofheat to promote the bonding material to bond between the substrate andthe at least one flexible circuit cable, wherein the columnar structurerestricts dispersion of the bonding material.
 2. The system of claim 1,wherein another plurality of gold bumps are bonded to another set ofinterconnection pads to create additional columnar structures, furthercomprising: a bending tool; at least another flexible circuit cable,wherein the bending tool bends the at least another flexible circuitcable with respect to the at least one flexible circuit cable andwherein bonding material is dispensed on the at least another flexiblecircuit cable; the alignment device aligning the substrate and the atleast another flexible circuit cable; the weight forcibly pressing thesubstrate and the at least another flexible circuit cable together; andthe HARS applying a second set of predetermined levels of heat topromote the bonding material to bond between the substrate and the atleast another flexible circuit cable.
 3. The system of claim 2, whereinthe at least one flexible circuit cable and the at least anotherflexible circuit cable are overlapping.
 4. The system of claim 2,wherein the first predetermined levels of heat and the secondpredetermined levels of heat are controlled to affect bonding betweenthe substrate and the at least one flexible cable and between thesubstrate and the at least another flexible circuit cable withoutaffecting electronic components.
 5. The system of claim 4, wherein thesecond predetermined levels of heat are controlled to affect bondingbetween the substrate and the at least another flexible cable withoutaffecting electronic components and the bonding between the substrateand the at least one flexible circuit cable.
 6. The system of claim 2,wherein the weight is cantilevered off of at least the at least oneflexible circuit cable and the at least another flexible circuit cableto enhance compression and bonding.
 7. The system of claim 2, whereinthe bonding material is dispensed at least twice on at least one of theat least one flexible circuit cable and the at least another flexiblecircuit cable.
 8. The system of claim 2, wherein an underfill is appliedbetween the substrate and the at least one flexible circuit cable andbetween the substrate and the at least another flexible circuit cable toprovide mechanical strength.
 9. The system of claim 2, wherein theplurality of gold bumps and the another plurality of gold bumps are laidin a predetermined pattern.
 10. The system of claim 2, wherein the HARSincludes a plurality of heaters to provide the first set ofpredetermined levels of heat and the second set of predetermined levelsof heat.